This invention relates generally to the field of respiration apparatus, and related methods for its use, in which a predetermined volume of gas is delivered to a patient in each breath, as in the administration of intermittent positive pressure breathing therapy. More particularly, the invention relates to a method and apparatus for the determination of the patient-dependent parameters of lung pressure, compliance and resistance, so that these parameters can be continuously displayed to the operator of the respiration apparatus.
Respiration apparatus used in administering intermittent positive pressure breathing therapy is well known in the art. In this type of therapy, a predetermined volume of air, or other breathable gas mixture, is delivered to the patient during an inspiration phase of the breathing cycle, and, during an expiration phase, the patient exhales against a pressure which may or may not be equal to atmospheric pressure, depending upon the exact nature of the therapy being provided. Since the predetermined volume is usually measured by means of a cylinder within the apparatus, and since part of this volume inevitably does not reach the patient but is retained in delivery tubes within the apparatus, it is well known that the volume actually reaching the patient, known as the tidal volume, will not always be the same as the volume measured by the cylinder in the apparatus. Moreover, the actual volume delivered will depend on the delivery pressure and various other factors. Consequently, some respirator systems include means for compensating for these errors in delivered volume, and for providing the desired volume, referred to as the compensated volume, to the patient, regardless of variations in delivery pressure. Such apparatus may be referred to as volume-compensated respiration apparatus. The present invention is principally concerned with the determination of a patient's lung pressure, compliance and resistance in conjunction with the administration of breathing therapy using volume-compensated respiration apparatus.
The patient resistance is a measure of the resistance to the flow of gas between the point of delivery of the gas to the patient, usually the mouth, and the patient's lungs. The measure of resistance is the pressure drop between the mouth and lungs divided by the volume rate of flow to the lungs, and the units of resistance are usually centimeters of water per liter per second. A high patient resistance may be indicative of a large blockage in the patient's mouth or trachea, and an increase in resistance may, for example, be indicative of a build up of mucus somewhere between the mouth and lungs.
The compliance of a patient's lungs is a measure of their resilience. The units of compliance are volume divided by pressure. Thus, a relatively compliant lung will expand to a large volume at a given pressure, while a lung of low compliance will be relatively stiff and unyielding as pressure is increased. Generally, an increase in patient lung compliance is indicative of an improvement of the patient's condition, but, in any event, the continued availability of both compliance and resistance information, and of lung pressure, is an extremely valuable tool to doctors or therapists treating patients under respiratory care.
The determination of patient compliance and patient resistance requires the measurement of the tidal volume. In volume-compensated systems, the tidal volume should be equal to the compensated volume, the value of which is known in such systems. The determination of compliance and resistance also requires the measurement of the patient lung pressure, as distinguished from system pressure at the point of delivery to the patient. Short of placing a pressure transducer in the lung, the accurate measurement of lung pressure has not been possible with available systems. Accordingly, respirator systems available heretofore have not provided for the accurate determination of patient resistance and compliance. It will be appreciated from the foregoing that a need exists for such a system. The present invention fulfills this need.